Okay so I'm a recentish convert, and I'm only intimately familiar with her last two albums.
Her very best song is: All Too Well
(It's about Jake Gyllenhaal)
The best song on 1989 is: Blank Space (by far!!!)
The second best song on 1989 is: I Wish You Would/Out of the Woods/Wonderland/New Romantics (ask me on different days and I'll give you a different answer)
Also okay on 1989: Shake It Off/Style (this one is about Harry Styles. Wonderland is also about Harry Styles probably but it's not as fun to say it)/All You Had To Do Was Stay/How You Get the Girl
The best song on Red is: All Too Well
The best song on Red if you like Snow Patrol is: The Last Time
The second best song on Red is: 22/I Knew You Were Trouble/We Are Never Ever Getting Back Together
The best song from the pre-Red era is: Mean? I guess? Maybe it's Mine